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    <title>Document</title>
    <style>
        .color{
            background-color: red!important;
        }
    </style>
</head>
<body>
<div>
    <ul id="geo">
        <li>
            <div>fhsdfhjsdfhsju</div>
            <div>fhsdfhjsdfhsju</div>
            <div>fhsdfhjsdfhsju</div>
            <ul>
                <li>fdnjfnjdnj
                <span>hfjfksdjfkr</span>
                    <div class="dd">
                        你给我变红
                    </div>
                </li>
            </ul>
        </li>
        <li>
            <div>fhsdfhjsdfhsju</div>
            <div>fhsdfhjsdfhsju</div>
            <div>fhsdfhjsdfhsju</div>
            <ul>
                <li>fdnjfnjdnj
                    <span>hfjfksdjfkr</span>
                    <div class="dd">
                        你给我变红
                    </div>
                </li>
            </ul>
        </li>
        <li>
            <div>fhsdfhjsdfhsju</div>
            <div>fhsdfhjsdfhsju</div>
            <div>fhsdfhjsdfhsju</div>
            <ul>
                <li>fdnjfnjdnj
                    <span>hfjfksdjfkr</span>
                    <div class="dd">
                        你给我变红
                    </div>
                </li>
            </ul>
        </li>
        <li>
            <div>fhsdfhjsdfhsju</div>
            <div>fhsdfhjsdfhsju</div>
            <div>fhsdfhjsdfhsju</div>
            <ul>
                <li>fdnjfnjdnj
                    <span>hfjfksdjfkr</span>
                    <div class="dd">
                        你给我变红
                    </div>
                </li>
            </ul>
        </li>
    </ul>
</div>
<script src="./jquery.js"></script>
<script>
$(function () {
    $("#geo li").eq(0).find(".dd").addClass("color");
})
</script>
</body>
</html>